Browse Prior Art Database

Transistor Current Sense for Switching Power Supply

IP.com Disclosure Number: IPCOM000046136D
Original Publication Date: 1983-Jun-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Kinnard, JR: AUTHOR

Abstract

The magnitude of current in a base drive inductor of a switching power supply is monitored as a current sensing circuit to protect a switching transistor from unacceptably high currents during start-up or overload conditions.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 54% of the total text.

Page 1 of 2

Transistor Current Sense for Switching Power Supply

The magnitude of current in a base drive inductor of a switching power supply is monitored as a current sensing circuit to protect a switching transistor from unacceptably high currents during start-up or overload conditions.

Switching power supplies normally require some form of current sensing circuit to protect the main switching transistor(s) from high currents during start- up or overload conditions. Two common methods of sensing transistor current require a resistor or current transformer in series with the emitter of the transistor. The sensing resistor value is a compromise between an acceptable signal level and power dissipation in the resistor, while the sense transformer is undesirable due to the expense of an additional magnetic component. Both methods require a delay or latch circuit to remember that the current threshold has been reached since the emitter current decreases by at least the amount of the base current before the transistor actually turns off.

As noted in U. S. Patent 3,925,717, a base drive inductor can be used to cause transistor base current to be a nearly constant percentage of collector current in a return-to-zero flyback-type power supply. Detecting the inductor current level instead of emitter current to protect the switching transistor has the following advantages: 1. Power dissipation is one-sixth as much since the

base current is typically 17% of emitter current.

2. The need for a delay or latch...